When a television set is switched from the "on" mode to "standby" mode or OFF, a ghost image may appear on some of the used cathode ray tubes (in the following abbreviated as CRT). This effect is called stray emission or after-glow and is normally caused by particles, in other word contamination, trapped at Grid 1 or at Grid 2 of the electron mount, i.e., electron guns, accelerated by the anode voltage (referenced as EHT for extremely high voltage) and hitting on the phosphor of the CRT. Due to the current CRT manufacturing process, it is not possible to eliminate these particles completely. CRT manufacturers normally guarantee that stray emission is suppressed if the EHT voltage is discharged immediately to a specified level (e.g., 18 kV) when the TV set is switched from on to standby.
A couple of methods to discharge the EHT voltage after switching the TV set to "standby" mode or OFF are known.
EP-OS 0 521 378 A discloses a television apparatus including a kinescope having a high voltage supply, a deflection circuit, a cathode driver amplifier and a control circuit for controlling operating modes of the receiver. A first transistor responsive to the turn-off command signal from the control circuit disables the high voltage supply and initiates turn-off of the deflection circuit at a controlled rate. Concurrently, a second means responsive to the turn-off command applies a "turn on drive" signal to at least one of the electron guns of said kinescope for discharging high voltage therefrom. Therefore, during turn-off the high voltage is drained from the kinescope as the display raster collapses thus providing features such as the safety feature of having a fully discharged kinescope, avoidance of kinescope spot burn and preventing of kinescope after-glow.
U.S. Pat. No. 5,266,870 discloses a stray emission prevention circuit for a system having a cathode ray tube including a charging and discharging section for applying a system power supply voltage upon power-on of the system to charge with a constant voltage and for discharging the charged voltage upon power-off of the system, and a switching section for forming an open circuit between a high voltage unit of the cathode ray tube and a ground upon the power-on of the system and forming a closed circuit there between upon the power-off of the system to discharge the residual high voltage in the cathode ray tube, the circuit being provided on a circuit board of the system.
The above methods for preventing stray emission have the disadvantages that they need an extra amount of circuitry which increase complexity and costs.
Further, the most popular and no-hassle method used is to install a bleeder resistor at the fly-back (in the following referenced as FBT) to discharge the EHT voltage transformer, as it will be explained later with the aid of FIG. 6.
Even though the use of a bleeder resistor has its merits, but one of the main disadvantages is the price of having the bleeder resistor installed in the FTB.
It is therefore an object of the present invention to provide a stray emission reduction circuit which is reliable and cost saving.
This object is solved by the features of the present claims. Preferred embodiments are subject of the independent claims.